Electric machines are used, for example, in hybrid vehicles, where they are operated either in the motor mode or in the generator mode. In the motor mode, the electric machine generates an additional drive torque, which supports an internal combustion engine, for example, in an acceleration phase; in the generator mode, it generates electrical energy, which is stored in an energy storage device such as a battery or a super-cab. The operating mode and power of the electric machine are set with the aid of an inverter.
Since in hybrid motor vehicles relatively high currents and voltages must be provided in order to be able to use the electric machine as a motor for driving the motor vehicle, operating parameters such as the phase currents of the electric machine are constantly monitored in order to protect the electrical system and the components connected to the electrical system.
When a malfunction, for example, a phase overcurrent, is recognized, the pulse-controlled inverter is switched into a safe state in order to prevent possible damage to the electrical components. Essentially two different switch-off methods are known from the related art. In a first method, all switches connected to a low potential (low-side switches) of the pulse-controlled inverter are closed and all switches connected to a high potential (high-side switches) of the pulse-controlled inverter are open. This operating mode is also referred to as short-circuit mode. In another switch-off method, all switches of the pulse-controlled inverter are open. This is also referred to as isolation mode.
A method is discussed in DE 10 2006 003 254 A1 for switching off an electric machine using pulse-controlled inverters in the event of a fault provides for minimizing undesirable side effects when switching off the electric machine and maximizing the regular machine operation by switching the electric machine first into an isolation mode, in which all switches of the pulse-controlled inverter are open, and subsequently into a short-circuit mode, in which the switches connected to the high potential are open and the switches connected to the low potential are closed.
A method for handling a fault in electric machines of a hybrid drive is discussed in DE 10 2007 020 509 A1 in which it is initially detected whether at least one operating parameter of the electric machine is greater than an associated operating parameter limiting value. If it is detected that at least one operating parameter is greater than the associated limiting value, a supply connection of the electric machine is activated for a time interval associated with the operating parameter and the supply connection is short-circuited to ground after the elapse of the time interval.